1. Field of Invention
The present invention relates to a development method that can be applied to an electrophotographic image forming apparatus such as a copy machine, printer, or facsimile device, and further relates to an image forming apparatus. More specifically, the present invention relates to a method for developing an electrostatic latent image that is formed on the surface of an electrostatic latent image bearing member with a toner, and further relates to an image forming apparatus.
2. Related Art
In an electrophotographic image forming apparatus, a development method has been adopted in which an electrostatic latent image is developed so as to make the image visible (developing) by electrically charging the surface of an electrostatic latent image bearing member (such as a photoreceptor) and forming the electrostatic latent image by exposing the image to the electrically charged region.
As such a development method, a development method has been used in which, using a single-component developer containing a toner or a two-component developer containing a carrier and a toner, by frictionally charging the toner so that the toner is attracted with the electrostatic force of an electrostatic latent image on the surface of the electrostatic latent image bearing member, the electrostatic latent image is developed, thus forming a toner image.
For example, when a two-component developer is used, a method has been adopted in which a magnetic brush is formed with the carrier on a developer bearing member (such as development roller) in a developing apparatus and an electrostatic latent image is developed by applying a bias voltage between the developer bearing member and an electrostatic latent image bearing member.
Also, regardless of whether the developer is single-component or two-component developer, development may be performed using a toner that is electrically charged with a polarity opposite to the surface potential of an electrically charged electrostatic latent image bearing member, or reversal development may be performed using a toner that is electrically charged with a polarity the same as the surface potential that is charged to the electrostatic latent image bearing member.
Furthermore, an electrostatic latent image that is formed on an electrostatic latent image bearing member may be developed with the toner by applying an oscillating bias voltage between the developer bearing member and the electrostatic latent image bearing member. In this oscillating bias voltage, a development-side electrical potential V2′ that can apply an electrostatic force to the toner to be electrically charged in the direction from the developer bearing member to the electrostatic latent image bearing member and an opposite development-side electrical potential V3′ that can apply an electrostatic force to the toner in the direction from the electrostatic latent image bearing member to the developer bearing member alternate with each other. For example, a rectangular wave is commonly used whose ratio (duty ratio) of the application time TH during which the development-side electrical potential V2′ is applied to the application time T of a cycle during which the development-side electrical potential V2′ and the opposite development-side electrical potential V3′ are applied is 50% (see FIG. 12 described later).
In such a conventional development method, it is desirable that the electrical charge amount of the toner is increased so as to achieve a smooth image quality with little roughness. However, for example, when a two-component developer is used, the electrostatic force between the carrier and the toner is in proportion to the square of the electrical charged amount. Accordingly, when the electrical charge amount of the toner is increased, the separation rate of the carrier from the toner decreases. As a result, the utilization efficiency of the toner deteriorates and the image density is reduced. In order to increase the image density, the Vpp (peak-to-peak voltage) of the oscillating bias voltage can be increased. However, if the Vpp is increased, so-called dot reproducibility, which is the ease of toner movement to a region that corresponds to an image portion in a region that occupies a large percentage of a region that corresponds to a non-image portion of the electrostatic latent image bearing member, tends to deteriorate.
As a method that achieves not only dot reproducibility but also improved image density, a method has been proposed in which the duty ratio of the oscillating bias voltage is varied. For example, in Japanese Patent 2933699 (published on May 11, 1992), in the oscillating bias voltages, the development-side electrical potential is increased and the application time of the development-side electrical potential is decreased while the opposite development-side electrical potential is decreased to prevent toner separation in a region that corresponds to an image portion and the application time of the opposite development-side electrical potential is increased. By adopting such a configuration, the toner that attaches to the region that corresponds to a non-image portion can be easily removed and the toner in the region that corresponds to an image portion including an intermediate tone portion can be retained.
However, when the inventor of the present invention conducted an experiment, it was found that the image density could not be sufficiently improved while maintaining good dot reproducibility when the duty ratio of the oscillating bias voltage is varied.
FIG. 12 shows rectangular waves with a duty ratio of 50%, and represents a bias waveform of a conventional oscillating bias voltage that is applied between a developer bearing member and an electrostatic latent image bearing member when reversal development is performed with a toner that is electrically charged with a polarity the same as the surface potential that is charged to the electrostatic latent image bearing member. Also, FIG. 13 is a graph that illustrates the relationship between the Vpp (peak-to-peak voltage) of the oscillating bias voltage shown in FIG. 12 and the image density.
As shown in FIG. 12, with respect to the conventional oscillating bias voltage that has rectangular waves of a duty ratio of 50%, when the Vpp is varied with a frequency of 5 kHz, the image density can be changed as shown in FIG. 13. In other words, the Vpp can be increased in order to improve the image density.
With the same conditions, the inventor studied variations in the dot diameter by forming a one-dot image at every eight dots, and found that the dot diameter decreases as the Vpp increases as shown in FIG. 14. Also, observation of each dot revealed that dot omission occurs more easily when the Vpp is large.
Therefore, improvement in the image density and dot reproducibility cannot be achieved at the same time since although the Vpp of the oscillating bias voltage can be increased to increase the image density, this deteriorates the dot reproducibility.
Next, with the oscillating bias voltages shown in FIG. 12, the inventor studied variations in density when an application time TH during which the development-side electrical potential V2′ is applied is varied while maintaining a constant application time T3′ during which the opposite development-side electrical potential V3′ is applied, and found that the image density significantly decreases as the application time TH of the development-side electrical potential V2′ decreases as shown in FIG. 15. In other words, it is difficult to maintain the image density since, when the duty ratio of the oscillating bias voltage is varied, the image density tends to decrease as the application time of the development-side electrical potential V2′ decreases.
If the absolute value of the development-side electrical potential V2′ is increased so as to supplement a decrease in the application time TH of the development-side electrical potential V2′, the image density can be slightly increased in some conditions. However, a significant increase in the image density cannot be expected. For example, when the application time TH of the development-side electrical potential V2′ is significantly shortened, the image density, on the contrary, decreases even when the application voltage of the development-side electrical potential V2′ is increased.